Maths links?

I'm coding a feature-light clone of Star Castle at the moment, but it's stretching my rather appaling maths abilities past their limit; so, anybody know of a good online resource for brushing up on maths skills?

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My stumbling block at the moment is determining the changes to make to the player's velocity, from the direction they're facing when thrusting - I know I could just code an if() containing some basic trigonometry, but I'd rather take the shorter, maths-only, path...